In general, in order to manufacture a desired semiconductor integrated circuit, various thermal processes including a film-forming process, an etching process, an oxidation process, a diffusion process, a modifying process or the like are carried out to a semiconductor wafer, which consists of a silicon substrate or the like. For example, as an oxidation process, there are known an oxidation process that oxidizes a surface of a single-crystal silicon film or a poly-silicon film, and another oxidation process that oxidizes a metal film, and so on. Such an oxidation process is mainly used for forming an insulation film such as a gate oxide film or a capacitor.
In view of a pressure, there are a normal-pressure oxidizing method that is carried out in a processing container under an atmosphere substantially the same as the atmospheric pressure, and a reduced-pressure oxidizing method that is carried out in a processing container under a vacuum. In addition, in view of a kind of gas used for the oxidation process, there are a wet oxidizing method that uses moisture vapor generated by burning hydrogen and oxygen by means of an outside burning unit (for example, JP A 3-140453), and a dry oxidizing method that causes only ozone or oxygen to flow into a processing container without using moisture vapor (for example, JP A 57-1232).
Herein, taking into consideration film characteristics as an insulation film such as pressure resistance, corrosion resistance, reliability or the like, in general, an insulation film formed by a wet oxidizing process is superior to an insulation film formed by a dry oxidizing process.
In addition, in general, an oxide film formed by a wet oxidizing process under a normal pressure can achieve a great oxidation rate, but is inferior in uniformity within a surface of a film thickness. On the other hand, an oxide film formed by a wet oxidizing process under a reduced pressure can achieve only a small oxidation rate, but is superior in uniformity within a surface of a film thickness.
Conventionally, design rules for a semiconductor integrated circuit were not so severe. Thus, the above various oxidizing methods were suitably selected taking into consideration use application of the oxide film, process condition for forming the oxide film, apparatus cost for forming the oxide film or the like.
However, recently, a wire width and/or a film thickness have been decreased so that the design rules for a semiconductor integrated circuit have become more severe. Thus, better film characteristics and/or higher uniformity within a surface of a film thickness have been required. The conventional oxidizing methods can not cope with the requests sufficiently.
An example of a wet oxidizing method is disclosed in JP A 4-18727. In this example, an H2 gas and an O2 gas are separately introduced into a lower-end portion of a longitudinal quartz reaction tube, and then burned at a burning part provided in a quartz cap. Moisture vapor is generated by the burning reaction. The moisture vapor rises up along an arrangement direction of wafers and oxidizes the wafers. In the case, the H2 gas is burned at the burning part, so that the lower-end portion of the processing container becomes rich in the moisture vapor. As the moisture vapor rises up, the moisture vapor is consumed. Thus, to the contrary, an upper-end portion of the processing container becomes short in the moisture vapor. Thus, film thicknesses of the oxide films formed on the wafers may be greatly different depending on supporting positions of the wafers. That is, uniformity between surfaces of a film thickness of the oxide films may be deteriorated.
In addition, in an oxidizing unit disclosed in JP A 57-1232, a plurality of semiconductor wafers are arranged alongside in a horizontal batch type of reaction tube. An O2 gas may be solely introduced into an end portion of the reaction tube. Alternatively, an O2 gas and an H2 gas may be simultaneously introduced thereinto. Then, an oxide film is generated under a reduced-pressure atmosphere. However, in the conventional unit, the film-forming process is carried out by using a hydrogen-burning oxidizing method under an atmosphere whose pressure is relatively high. That is, the moisture vapor is main in the reaction. Thus, as described above, density difference of the moisture vapor may be generated between an upstream side and a downstream side of the gases in the processing container. Thus, uniformity between surfaces of a film thickness of the oxide films may be deteriorated.
In addition, in a unit disclosed in the specification of U.S. Pat. No. 6,037,273, an oxygen gas and a hydrogen gas are supplied into a single-wafer type of process chamber, which is heated by a lamp. The both gases react in the vicinity of a surface of a semiconductor wafer arranged in the process chamber so as to generate moisture vapor. The moisture vapor oxidizes silicon of the wafer surface, so that an oxide film is formed.
However, in the unit as well, the oxygen gas and the hydrogen gas are introduced into the process chamber from gas ports which are located away from the wafer by about 20 to 30 mm, and the process pressure is relatively high. Thus, uniformity within a surface of a film thickness is inferior.
In order to solve the above problems, JP A 2002-176052 by the applicant discloses an oxidizing method wherein an oxidative gas such as an O2 gas and an reducing gas such as an H2 gas are simultaneously supplied into an upper portion and a lower portion of a process chamber respectively, and react on each other under a vacuum atmosphere in order to form an atmosphere mainly consisting of oxidation active species and active hydroxyl species. In that atmosphere, the silicon wafer or the like may be oxidized.
The oxidizing method is explained simply with reference to FIG. 11. FIG. 11 is a schematic structural view showing an example of a conventional oxidizing unit. The oxidizing unit 202 shown in FIG. 11 has a longitudinal cylindrical processing container 206. A resistance heater 204 is arranged around the processing container 206. In the processing container 206, a wafer boat 208 is arranged, which can be moved up and down in order to be loaded and unloaded through a lower end of the processing container 206. Semiconductor wafers W consisting of silicon substrates or the like are placed and held on the wafer boat 208 in a tier-like manner. An H2-gas nozzle 210 for supplying an H2 gas and an O2-gas nozzle 212 for supplying an O2 gas are provided at a lower side wall of the processing container 206. A gas-discharging port 214 connected to a vacuum pump not shown or the like is provided at an upper portion of the processing container 206.
The H2 gas and the O2 gas introduced into (a lower portion of) the processing container 206 from the both nozzles 210, 212 react on each other in the processing container 206, for example under a pressure smaller than 133 Pa, in order to generate active oxygen species and active hydroxyl species. These active species rise up in the processing container 206, come in contact with surfaces of the wafers W, and oxidize the surfaces.
According to the oxidizing methods disclosed in the above six documents, an oxide film having good film characteristics can be formed, and the uniformity within a surface of a film thickness of the oxide film can be maintained high.
Herein, recently, for a case wherein different materials are exposed on a surface of a semiconductor wafer, it has been requested that an oxide film having a good film quality is selectively formed on the wafer surface. For example, when a semiconductor integration circuit having a gate structure consisting of an ONO film, such as a flush memory, is manufactured, under a situation wherein both a silicon layer and a silicon nitride layer are exposed on a surface of a semiconductor wafer, it has been requested that an oxide film having a good film quality is selectively formed only on a silicon layer while an oxide film of SiO2 is formed on the silicon nitride layer as little as possible. In the case, according to the above method using the active oxygen species and the active hydroxyl species in a simple manner, the oxidative effect is so strong that an oxide film of SiO2 having a considerable thickness may be formed not only on the silicon layer but also on the silicon nitride layer. That is, a sufficient selective oxidation process can not be achieved.
The above point is explained in more detail with reference to FIGS. 12A to 12C. Each of FIGS. 12A to 12C is a view showing a part of a manufacturing method of a semiconductor integration circuit having a gate structure consisting of an ONO film. As shown in FIG. 12A, for example, in a gate structure for a flush memory, a first gate electrode 104 consisting of poly-crystal silicon is formed on a silicon substrate 100 via a gate oxide film 102. On the first gate electrode 104, an ONO film consisting of a three-layer structure of a silicon oxide film 106, a silicon nitride film 108 and a silicon oxide film 110 is formed.
During a forming step of such a gate structure, a gate oxide film 112 for a peripheral circuit element may be formed (see FIG. 12B). In the case, an oxidation process is conducted for forming the gate oxide film 112. Then, after the oxidation process, an electrode-forming process is conducted, so that a gate oxide film 112 for a peripheral circuit element is formed at the same time that a second gate electrode 114 for a flush memory is formed, as shown in FIG. 12C.
Herein, for the oxidation process for forming the gate oxide film 112 as shown in FIG. 12B, if a conventional low-pressure active-species oxidizing method that uses the active oxygen species and the active hydroxyl species in a simple manner is adopted, the silicon oxide film 110 at the uppermost part of the ONO film absorbs silicon atoms from the silicon nitride film 108 to be thickened, so that the silicon nitride film 108 may be made thinner. That is, there is a possibility that a designed ONO-film structure is not obtained.